全文获取类型
收费全文 | 5350篇 |
免费 | 394篇 |
国内免费 | 1篇 |
专业分类
5745篇 |
出版年
2023年 | 14篇 |
2022年 | 44篇 |
2021年 | 80篇 |
2020年 | 43篇 |
2019年 | 48篇 |
2018年 | 55篇 |
2017年 | 65篇 |
2016年 | 116篇 |
2015年 | 192篇 |
2014年 | 208篇 |
2013年 | 260篇 |
2012年 | 359篇 |
2011年 | 361篇 |
2010年 | 231篇 |
2009年 | 196篇 |
2008年 | 308篇 |
2007年 | 279篇 |
2006年 | 267篇 |
2005年 | 302篇 |
2004年 | 253篇 |
2003年 | 283篇 |
2002年 | 266篇 |
2001年 | 80篇 |
2000年 | 54篇 |
1999年 | 86篇 |
1998年 | 87篇 |
1997年 | 67篇 |
1996年 | 52篇 |
1995年 | 49篇 |
1994年 | 54篇 |
1993年 | 57篇 |
1992年 | 76篇 |
1991年 | 51篇 |
1990年 | 47篇 |
1989年 | 38篇 |
1988年 | 52篇 |
1987年 | 42篇 |
1986年 | 28篇 |
1985年 | 47篇 |
1984年 | 50篇 |
1983年 | 30篇 |
1982年 | 37篇 |
1981年 | 47篇 |
1980年 | 35篇 |
1979年 | 27篇 |
1978年 | 32篇 |
1977年 | 27篇 |
1976年 | 21篇 |
1974年 | 23篇 |
1961年 | 11篇 |
排序方式: 共有5745条查询结果,搜索用时 0 毫秒
21.
22.
23.
It is common in community ecology to use the species as the taxonomic category of interest, yet in rich tropical assemblages containing guilds of very similar species this may not be appropriate. Such assemblages may be organized at the level of guilds rather than at the finer species level. In a ten-year study of assemblages of fish at One Tree Reef, Great Barrier Reef, we found species composition and the number of fish on a given lagoonal patch reef vary greatly across time (Sale and Douglas 1984; Sale et al. in preparation). The mean average proportional similarity of a reef's assemblage to itself at different times (censuses) is usually low at a value of around 0.5. This apparent variability may be ecologically irrelevant noise if organization is at the higher guild level. We have recast our database at the guild level to test this possibility. Thirteen guilds were defined by the diets, foraging habitats and times of the individual species comprising them. Similarity of an assemblage to itself at successive censuses was re-calculated using the number of individuals in each guild instead of the numbers in each species. This analysis yielded significantly higher levels of similarity (P<0.01) among censuses. To test whether this increase in similarity was due solely to the smaller number of categories used to calculate the similarity indices, 5 sets of randomly generated guilds were constructed using a Monte Carlo approach. No significant difference (P>0.05) was found between the average similarity among censuses when assemblages were classified into these null guilds and when they were classified according to the real guilds. These results indicate that shifting to the larger taxonomic scale of guilds does not reveal a significantly more persistent assemblage structure than that revealed when analysis is at the smaller scale of species. There is no evidence of an underlying organization of these assemblages at the guild level. 相似文献
24.
Grard Febvay Jean-Franois Pageaux Guy Bonnot 《Archives of insect biochemistry and physiology》1992,21(2):103-118
Polar and neutral lipids and their constitutive fatty acids were quantified in the pea aphid, Acyrthosiphon pisum (Harris), grown on host plant or on a lipid free artificial diet. The results were compared to determine if lipids were involved in the suitability of the diet for continuous rearing of this A. pisum biotype. For apterous adults grown on plants, the lipids were characterized by a low amount of neutral lipids (2.5% weight/fresh weight) almost entirely (96.4%) composed of hexanoyl and sorboyl dimyristin. These storage lipids were higher in the alatae (3.8%), probably correlated with potential flight activity. The phospholipid amounts were identical in these two morphs (1.3–1.4% weight/fresh weight), comprised mainly of phosphatidylethanolamines (52%) and phosphatidylcholines (40.6%). These phospholipids contained a still unidentified fatty acid, with a retention time close to that of linolenic acid and synthesized by the aphid or its bacterial symbionts (not found in plants). The apterous adult aphids reared on an artificial diet showed an accumulation of neutral lipids (8.9% for the first generation); this increase was shown to be slightly greater for the hexanoyl and sorboyl triglycerides. In contrast, the phospholipids decreased in aphids reared on an artificial diet (1.1% and 0.9%, respectively, for first and second generation), correlated with a phospholipid fatty acid profile significantly deficient in C18:3 and in the unidentified peculiar fatty acid. These phospholipids are essential components of biological membranes and a diet-driven deficient synthesis in some of their components may result in the observed symptoms. © 1992 Wiley-Liss, Inc. 相似文献
25.
Structure and in vitro replication of DNA templates containing 7,8-dihydro-8-oxoadenine. 总被引:5,自引:4,他引:1 下载免费PDF全文
Adenine residues in DNA are oxidized under the action of ionizing radiation at the C-8 position to give 7,8-dihydro-8-oxoadenine. The formation of this lesion can be considered a cause of mutations and carcinogenesis. Oligodeoxyribonucleotides 39 and 47 bases long containing a single 7,8-dihydro-8-oxoadenine (8-hydroxyadenine) residue were synthesized by using nucleoside phosphoramidites. They were used as templates to study the copies obtained in vitro by the Klenow fragment and the thermostable Taq DNA polymerase. 7,8-Dihydro-8-oxoadenine does not block the replication and thymine is incorporated opposite the damage. The modifications of the DNA duplex conformation provoked by 7,8-dihydro-8-oxoadenine are minor. 1H-NMR spectroscopy shows that the duplex is in a B form, the sugar in a normal position in the helix and the modified base in the anti position. NMR confirms that 7,8-dihydro-8-oxoadenine exists predominantly in the keto form. 相似文献
26.
The solution structure of a DNA hairpin containing a loop of three thymidines determined by nuclear magnetic resonance and molecular mechanics. 总被引:7,自引:6,他引:1 下载免费PDF全文
Y Boulard J Gabarro-Arpa J A Cognet M Le Bret A Guy R Toule W Guschlbauer G V Fazakerley 《Nucleic acids research》1991,19(19):5159-5167
We have determined by two-dimensional nuclear magnetic resonance studies and molecular mechanics calculations the three-dimensional solution structure of a 21 residue oligonucleotide capable of forming a hairpin structure with a loop of three thymidine residues. This structure is in equilibrium with a duplex form. At 33 degrees C, low ionic strength and in the presence of MgCl2 the hairpin form dominates in solution. Six Watson-Crick base pairs are formed topped by the loop structure. The residues 1-3 and 18-21 are not complementary and form dangling ends. Distance constraints have been derived from nuclear Overhauser enhancement measurements. These, together with molecular mechanics calculations, have been used to determine the structure. We do not observe stacking of thymidine residues either over the 3' or the 5' end of the stem. 相似文献
27.
We studied the stability of the genomic distribution of six retrotransposon families in long-term and short-term cultures of Drosophila cells. In a subclone derived from Kc cells, no significant rearrangements were detected over an 8 year period. On the contrary, extensive reshuffling and amplification of transposon families were observed in recently established cell lines. These results show that in cultured Drosophila cells transposition appears to be restricted to the transition from the embryo to continuous cell lines. 相似文献
28.
Both positive and negative biological effects of microwaves on drug actions in rats exposed to 1-mW/cm2, 2,450-MHz microwaves have been reported by several investigators. We conducted dosimetry studies for seven different exposure conditions to determine whether these different results could be due to the rats having been exposed differently. They included anterior and posterior exposures in a circular waveguide, near field, far field with E- or H-field parallel to the long axis of the body and dorsal exposure in a miniature anechoic chamber with E- or H-field parallel to the long axis of the body. The average specific absorption rates (SARs) in the head, tail, and body of the exposed rats were measured by means of a calorimetry system. The local SARs at eight locations in the brain were determined by temperature measurement with Vitek probes. Intensive coupling of energy to the tail when it was exposed parallel to the E-field was shown by thermography. For the same average incident power density, the average SARs in the heads of rats were about two times higher in the circular waveguide than for other exposures. The local SARs in the brain varied for different exposure conditions. Statistical comparisons of SARs under the different exposure conditions are presented. 相似文献
29.
Guy L. Nesom 《Brittonia》1984,36(4):396-401
Chaptalia dentata (L.) Cass. andC. albicans (Sw.) Vent. ex Steudel incorrectly have been recognized as conspecific in recent treatments. Although they are vegetatively similar, they differ in features of the flowers and fruits—especially the mature achenes. Both species occur in the Bahamas and the Greater Antilles of the West Indies:Chaptalia dentata is endemic to that area, absent only from Jamaica;C. albicans is more widespread, as it occurs in Jamaica as well as in scattered localities on the American mainland—southern Florida, various localities in Belize, Honduras, and Guatemala, and the Mexican states of Veracruz, San Luis Potosí, Chiapas, and Yucatán. 相似文献
30.
Glycinebetaine content of halophytes: Improved analysis by liquid chromatography and interpretations of results 总被引:2,自引:0,他引:2
The glycinebetaine content of plants can be determined by simple isocratic high performance liquid chromatography. The method is applicable to extracts from a wide range of species and, in most cases, is suitably rapid and specific to be preferable to other methods of analysis. The chromatographic system employed permits accurate and sensitive ultraviolet detection, free of most interferences. Because the principle plant carbohydrates elute well before glycine betaine, preparative ion exchange procedures can be simplified. Twenty-seven species, mostly inland halophytes, were screened by these methods and 13 were found to be glycinebetaine accumulators. On a dry weight basis, the glycinebetaine content of Salicornia europaea L. actually declined with exposure to progressively higher levels of NaCl. When expressed as a proportion of plant organic matter, however, patterns were more typical (up to 7.7% at higher salt concentrations). 相似文献